1. Field of Invention
The current invention concerns a process for selective thermal oxidation or photooxidation of hydrocarbons adsorbed onto zeolite matrices. In particular, the invention concerns highly selective thermal oxidation and photooxidation of unsubstituted or alkyl substituted alkanes, alkenes, aromatics and cycloalkyls in solvent free zeolites under dark thermal conditions or under irradiation with visible light. The process oxidizes hydrocarbons almost completely selectively without substantial production of byproducts.
2. Background and Related Disclosures
Partial oxidation of small alkanes, alkenes, and aromatics is one of the most important processes in chemical industry. The oxidated products serve as building blocks for plastics and synthetic fibers, or as industrial intermediates in the manufacture of fine chemicals. Oxidation of low alkanes plays a central role in the use of natural gas and volatile petroleum fractions as new feedstocks for industrial chemicals. For these large scale processes, molecular oxygen is the only economically viable oxidant. For most small hydrocarbons, direct oxidations by O.sub.2 are very unselective. As a result, existing methods generate large amounts of unwanted byproducts which require energy-intensive separation processes.
The main reason for the lack of selectivity is the free radical nature of the gas or liquid phase processes, the high exothermicity of the reactions, and overoxidation. Unrestricted mobility of the free radical intermediates results in indiscriminate attack on starting hydrocarbon and primary oxidation products. Overoxidation is due to the fact that, under thermal conditions in liquid or gas phase, oxygen attacks partially oxidized products more easily than the starting hydrocarbon. The lack of control gets worse as products accumulate, limiting conversion to a few percent in most practical processes.
Recent efforts towards improvement of the selectivity of hydrocarbon oxidation by O.sub.2 encompass a diverse spectrum of approaches. Low alkane oxidations are mainly based on catalysis over metal and mixed metal oxides (Appl. Catal. 128:L165 (1995)), with some of these solids acting mainly as oxidative dehydrogenation catalysts (Topics Catal., 3:277 (1996)). Mixed metal oxides were shown to play an important role in oxidation of unsaturated hydrocarbons (Appl. Catal. A., 143:29 (1996)). Some solid oxides have shown to be effective oxidation catalysts under irradiation with UV light (J. Chem. Soc. Chem. Commun., 2125 (1996)). Other methods, such as electrochemical methods (J. Catal., 157:450 (1995)) and catalysis by transition metal complexes (J. Am. Chem. Soc., 116:998 (1994)) are under investigation. Although selectivities are dramatically improved over plain autoxidation, these methods still generate substantial amounts of carbon oxides or other carbon fragmentation products, some of these being produced even at low hydrocarbon conversion. Porphyrin analogs of monooxygenase enzymes that are capable of low alkane and olefin oxidation by O.sub.2 to alcohols or epoxides are described in Metalloporphyrins in Catalytic Oxidations, R. A. Sheldon, Ed., Marcel Dekker, New York (1994). Many porphyrin systems require sacrificial reducing agents, but some afford oxidation of even small hydrocarbons without the need of a stoichiometric reductant. These include perhalo iron porphyrins (ibid.), UV light-assisted oxidation of alkanes in the presence of metalloporphyrins (J. Chem. Soc. Chem. Commun., 1487 (1991)) and epoxidation of olefins by Ru porphyrin (J. Am. Chem. Soc., 107:5790 (1985)). A most recent approach is oxidation in redox molecular sieves such as metal aluminophosphates or metal silicalites (Appl. Catal. A., 43:3 (1996)). Selectivities of these approaches are higher, but typically only at a few percent conversion of the hydrocarbon, a persistent problem in all existing methods using O.sub.2 that is especially severe for low alkanes and alkenes.
Charge-transfer from alkane (or alkene, aromat) to oxygen can be induced by absorption of a photon by a hydrocarbon.cndot.O.sub.2 collisional complex or, in principle, spontaneously in a thermal process if molecules were occluded in a highly ionic environment. Light-induced formation of hydrocarbon.cndot.O.sub.2 charge-transfer states has been reported in J. Am. Chem. Soc., 82:5966 (1960). Optical absorptions in the UV region originating from transition to excited charge-transfer states of alkane, alkene, or aromat.cndot.O.sub.2 collisional pairs were observed in O.sub.2.sup.- saturated hydrocarbon liquids and high-pressure O.sub.2 gas phase. They appear typically as long, structureless absorption tails. Upon irradiation with UV light, photooxidation was observed and interpreted by a mechanism that features proton transfer from the hydrocarbon radial cation to O.sub.2.sup.- as the initial step (Tetrahedron, 41:2215 (1985)). However, these UV light-driven gas or liquid phase oxidations resulted in a multitude of products and were therefore nonselective.
It would, therefore, be highly advantageous to provide a method or process which would selectively oxidate small hydrocarbons and which would achieve selective activation via charge-transfer between hydrocarbon and O.sub.2 - in such a way as to generate the radical cation.cndot.O.sub.2 - pair and also which would have some means to control the chemistry of the subsequently produced radicals and primary oxidation products.
All patents, patent applications and publications cited herein are hereby incorporated by reference.